There are many steps involved in the typical manufacturing process of semiconductor wafers. For example, the typical incoming wafer (e.g., bare silicon wafer) starts with a deposition of an insulating layer (i.e., a dielectric film, such as glass). After that, a layer of a pattern mask is laid down on the insulating layer using lithography. Then, materials are selectively removed from these layers using etching. After that, the photoresist mask is removed (i.e., strip) and the residues and particles are removed (i.e., clean or polish). Finally, conducting material is deposited (i.e., deposition) for each semiconductor device of the wafer. In short, the simplified and typical steps include deposition, lithography, etch, strip, clean, and deposition. Of course, frequently, many of these steps are repeated to form multiple layers.
According to their design, each layer has a pattern of material that is laid down in a manner so that they align with the corresponding material of the adjacent layer. For example, contacts of one layer align with those of the next. Misalignments can cause short circuits and connection failures that impact the effective yield and costs. The alignment of the pattern of materials of adjacent layers is called overlay herein.
The overlay presumes that each layer is perfectly flat or nearly so. However, it is common for the wafer not to be flat, and indeed the wafer may be significantly curved. This is called a wafer bow. Because the wafer is not flat, a layer on that bowed wafer is likewise not flat.
The bow of a semiconductor wafer is the deviation of the center point of the median surface of a free, un-clamped wafer from the median surface to the reference plane, where the reference plane is defined by three corners of the equilateral triangle.
The Detailed Description references the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.